A known disadvantage in batteries of all types is that their internal electrochemical reaction, and thus the maximum utilizable output, basically depends very heavily on the battery""s internal temperature. When the temperature drops, the speed of the chemical reaction in the electrons decreases. This lowers the maximum electrical current which the battery is able to supply during constant no-load voltage, and thus the battery""s output. Furthermore the speed of the mass transfer within the electrolyte and within the porous battery electrodes decreases. Both factors considerably increase the internal resistance of the cold battery. This means that in a supercooled condition of the cells even a fully charged battery is unable to supply all of its nominal electrical output because of its high internal resistance. However the nominal output can be obtained again without adding any charging energy as soon as the battery warms up to normal temperature.
The temperature dependence is also especially disadvantageous for example when the battery has to supply a D.C. voltage transformer where a predetermined electrical output is obtained from the secondary side, regardless of the voltage at the battery contacts. The transmission operation of a radio telephone for example requires an electrical output of a few watts for the power amplifier. Because of the high internal resistance, the voltage at the battery contacts is low and the transformer control causes an increase in current consumption in order to provide the required output. This in turn causes a further voltage reduction at the contacts. Since the transformer does not receive sufficient output, its control is interrupted and goes into an uncontrolled operating mode which corresponds to a total battery discharge.
For example if the transformer is used for a radio telephone, its control perceives this operating condition as a discharged battery and switches the radio telephone to the stand-by mode to protect it from becoming fully discharged. Although the battery is still sufficiently charged, it is impossible in this mode to establish a connection to the network. This situation represents a considerable safety risk for the emergency call function of the radio telephone.
It is known for example to protect motor vehicle starter batteries as long as possible against supercooling by adding a casing of a good thermal insulation material. Various sellers offer such a casing as a car accessory. However, on the one hand this requires a large additional volume and on the other it only provides a time-limited effect. The solution is especially unsatisfactory for a radio telephone because an additional casing undesirably increases the volume of the telephone.
To extend this effect, electronically controlled heating plates can be found in the auto accessories market. They are cemented to the surface of the starter battery housing and have electrical connectors which must be connected to the battery contacts. Such a heating plate prevents cooling of the battery by using its own power, so that its internal temperature remains in a range where the internal resistance is low enough so that the desired output can be obtained at any time.
For reasons of electrical insulation however, the cells of a battery are encased in a material which is also a good heat insulator. This causes a high thermal resistance between the inside of the battery and the heating plate and the battery""s environment is kept warm at a considerable cost of electric power to prevent a decrease in its internal temperature. Due to the high thermal resistance however, it is not possible to reactivate a supercooled battery with justifiable cost of time and energy. At low temperatures therefore the heating plate always supplies a significantly higher amount of power to the environment than is required by the inside. This can overtax the battery""s capacity. There is the danger that so much power has already been used to maintain the normal temperature that it is no longer possible to establish a connection because the battery is discharged.
It is also known from applicable safety provisions for the safe handling of batteries, such as the IEC recommendations for example, that an external short circuit of the connections of a battery must be strictly avoided. As can be found on the Varta Company internet site: xe2x80x9cBasics on the subject of batteries. Additional questions for advanced students, xe2x80x9chttp//www.varta.de/knowhow/100quest/100-003.html 7xe2x80x9d, an external short circuit can have serious consequences if high gas pressure builds up inside the battery.
To carefully charge a radio telephone""s battery it is known to place a temperature sensor in the battery between the insulating external skin and the metallic cell body. The radio telephone""s control circuit uses this sensor to determine the battery""s temperature with a relatively small delay because of the metallic contact, and then interrupts the charging if the battery has been heated to a predetermined degree. This protects against overcharging.
In the area of video technology, compare for example Philips"" correspondence lessons: Electrical technology and Electronics, Volume 2, Technique and Application, the heavily reworked 8th. issue, section xe2x80x9cHorizontal Deflection Stepsxe2x80x9d, pages 231 ff, Heidelberg: Hxc3xcthig, which describes a simple functional principle for producing a sawtooth-shaped alternating current for horizontal deflection, from a direct current source such as a battery. In principle the direct current source has an inductance in series with a switch, which is bridged by a diode and conducts and blocks with alternating current. A capacitance is in parallel with the inductance. With the appropriate choice of component values in relation to the switching times of the switch, the following takes place: During the time the switch is conducting, a current from the battery builds a magnetic field in the inductance. It collapses after the switch blocks the current, which reverses its direction and the battery power oscillates in the form of a resonance vibration half-wave between the inductance and the capacitance and back. During this half-wave the voltage amplitude is still positive. The subsequent negative half-wave opens the diode and with ideal, namely loss-free components, the battery power flows back into the battery. The process is therefore also called xe2x80x9cpower recoveryxe2x80x9d.
One advantage of the present invention is that it can be used regardless of cell type, cell size and the battery""s structural form. The invention is particularly suited for batteries which are used to supply power to mobile devices suc as radio telephones or radio device, since the device can operate again after a few minutes. This makes it possible, for example, to make an emergency call with a radio telephone in dangerous situations, even under the effect of extreme cold such as takes place in polar regions or in Alpine areas. The content xe2x80x9cradio telephonexe2x80x9d is used in the present case as a generic term for all types of devices for wireless communication, particularly mobile telephones, car telephones, satellite telephones, mobile fax machines and mobile computers which can communicate with a network.
Starting with the defects of the known solutions for reactivating a battery, the object of the invention is to create a solution for a radio telephone which, with a justifiable expense of time and energy enables the use of the radio telephone as fast as possible after the effect of extremely low temperatures, or keeps the telephone in operating condition.
The reactivation of the battery takes place according to the invention through the internal heating of the electrolyte, which is done with the aid of a time-controlled intrinsic current via the battery contacts, where the external circuit transforms a negligibly small electrical power. The high internal resistance, which operates as an internal heating element, is used against supercooling. The amount of electrical power drawn from the battery for that purpose is directly converted into heat inside the battery. The result is the significantly faster heating of the internal battery structure than with the known solutions, with minimum loss of power to the environment.
A short time after the start of the reactivation, the internal resistance decreases due to the rising temperature of the electrolyte. Therefore a control circuit can intelligently monitor this process and optimally control the current for reactivating the battery in accordance with its present condition.
The internal resistance of a supercooled battery is relatively large. Its battery contacts could therefore be continuously loaded by a short circuit during the reactivation, without suffering any damage. Because of the safety provisions, this supposedly simple solution is neither permissible nor suitable in practice, since during the short circuit there is no voltage at the battery connectors to operate a control circuit, which would interrupt the process in the presence of heat. This precludes any time control of the process.
It is therefore a further object of the invention to indicate a possibility of heating the inside of the battery without causing power losses that are worth mentioning in the external circuit, so that the battery contacts can supply a voltage for operating the control circuit.
The invention achieves this in that the external circuit periodically loads the battery with short-term current peaks at or near the level of the short-circuit current. In the simplest case this is done with a switch that bridges the battery connectors in accordance with a pulse frequency which advantageously has a load ratio that depends on the internal temperature. Outside of the switch""s conducting time, the battery loads a charging capacitor in order to provide operating voltage for the control circuit.
Investigations have shown however that an alternating current, which flows through the battery contacts during the reactivation, speeds up the process. For that reason and according to a first advantageous configuration of the invention, the external circuit for charging the battery is designed so that the electrical power from the battery oscillates periodically in both directions, thus like an alternating current, between the battery and an energy store in the external circuit. With this configuration as well, any active power is essentially only converted into heat inside the battery. To that end the external circuit contains a reactive (wattless) load with at least one inductive and/or capacitive element, whose connections are periodically switched by the control circuit via at least one switch, so that the reactive load is charged and discharged with alternating current.
According to another advantageous configuration of the invention, the control circuit utilizes the circumstance that the amplitude of the momentary battery voltage depends directly on the internal temperature, even during the time-limited consumption of a current. It must however be remembered that a measurement of the battery voltage while the battery contacts are under load does not provide any sure information about the battery""s internal temperature. A mostly discharged battery as well as a supercooled battery has a high internal resistance and can therefore not be distinguished from a supercooled but still fully charged battery. According to the invention however the control circuit reliably distinguishes a supercooled battery from one with a low charge, by evaluating the voltage at the battery contacts in conjunction with a signal provided by the above mentioned temperature sensor. This provides the control circuit with an additional indication of the battery""s temperature status during the reactivation. Since it indicates the result of the reactivation without any time delay, the reactivation ends as soon as the amplitude of the battery voltage exceeds a minimum value during the current peaks. In this way a supercooled battery can begin to be reactivated at a high load without causing any lasting damage and without violating relevant safety provisions. As soon as the internal resistance drops, the load can be reduced without any time delay.
Since in addition the time delay between the supply of power and the heating of the electrolyte is shorter than with the known solutions, the control circuit ensures that only as much power as needed to operate a radio telephone is taken from the battery.
In the following the invention will be explained by means of embodiments. The corresponding drawings show: